Myopia or short-sightedness is a problem of the eye wherein objects at a distance are focused in front of the retina, causing blurred vision; that is, the focusing power of the eye is too great. Myopia is normally corrected with the use of ophthalmic lenses of sufficient negative power to bring distant objects back into focus on the central retina, while allowing near objects to be focused on the central region of the retina by accommodation of the lens of the eye. Myopia is commonly a progressive disorder associated with gradual elongation of the eye so that lenses of increasing negative power are needed over time. A number of undesirable pathologies are associated with progressive myopia.
It is now generally accepted that elongation of the eye of a growing animal is normally controlled by a feedback mechanism that enables axial light rays entering the eye to be focused onto the central region of the retina. It is assumed that, with emmetropia, this mechanism works well but that, in myopia, the elongation is excessive while, in hyperopia, it is insufficient to allow good focus of the axial rays. Until the recent work of Smith and others (discussed in part in the aforementioned U.S. Pat. No. 7,025,460, and incorporated by reference herein, as if made a part of the present application), it was commonly accepted that the stimulus controlling the feedback mechanism had to do with features of the central image formed in the eye. Smith has now convincingly shown that the stimulus has little to do with the quality of the central image but is related to the curvature of field or peripheral refraction; that is, the quality of the peripheral image. More particularly, Smith demonstrated that a stimulus for increased eye length is created when the peripheral focal plane lies behind (is posterior to) the retina and that this condition may persist despite excessive and continuing growth of the eye from the standpoint of optimal central vision. Smith therefore proposed the use of corrective eye lenses for myopia that shift the focal plane in front of (anterior to) the peripheral retina. However, the lenses, especially contact lenses, suggested by Smith are difficult to design and manufacture, and may introduce noticeable visual distortion in peripheral vision.
Prior to the teachings of Smith, a variety of multi-focal contact lenses had been proposed on the common assumption that aspects of the eye's central image provide the stimulus for abnormal eye growth in myopia. Though such prior art is not of direct relevance to the present invention, those items considered to be of most interest are reviewed below.
U.S. Pat. No. 6,752,499 to Aller teaches prescribing commercially available bifocal contact lenses for young myopic patients who also exhibit near-point esophoria in the hope of controlling the progression of myopia. The preferred lenses were those with concentric near and distance zones within the normal pupil diameter of the patient. Such bifocal contact lenses had been designed and prescribed for the correction of presbyopia in older eyes. However, Aller proposed that they should be prescribed for the selected myopic patients to provide additional refractive power (myopic defocus) at both near and distance. Obviously these lenses have the inherent disadvantage that at least one out of focus axial image is present on the central retina at all times, degrading image quality for both distance and near gaze. Moreover, when the wearer is viewing a near object and the eye is making use of the near zone of the lens, the distance zone not only creates an unwanted and out-of-focus image of the object but, more significantly, portion of this out-of-focus image is likely to be present posterior to the peripheral region of the retina and, according to the teaching of Smith, to thereby provide a stimulus for myopia progression.
U.S. Pat. No. 6,045,578 to Collins et al. (Collins) teaches the addition of positive spherical aberration at the central retina in the hope of providing a stimulus that will reduce or control the progression of myopia on the basis that some positive spherical aberration is normally found in the emmetropic adult eye. This principle is applied to a variety of eye lenses including contact lenses. However, the deliberate introduction of spherical aberration into the central image degrades that image and visual acuity. Collins gives no attention to the nature of the image in the peripheral region of the retina where, as taught by Smith, the essential stimulus for eye growth is provided. Significant trial results using Collins-type lenses with deliberately introduced spherical aberration in the central image for the control of the progression of myopia have not been reported to the applicant's knowledge.
International Patent Application No. WO200604440A2 by Phillips et al. (Phillips) discloses the use of bifocal contact lenses in which there is (i) a vision correction area for correcting the myopic central vision of a wearer and (ii) a myopic defocus area which simultaneously presents a myopic defocused in the wearer's central vision at both near and distance gaze. Since (as is characteristic of multi-focal lenses) both areas of the lens fall within the normal pupil diameter of the patient, the same basic problem of degraded central image is also present here. Similar problems are evident with the teachings of US Patent Application No. 2006/0082729 by To, which discloses the use of multi-focal Fresnel contact lenses that provide myopic defocus in central vision, but they are exacerbated by the fact that Fresnel lenses degrade image quality relative to refractive lenses.